W-CDMA (Wideband Code Division Multiple Access) is a radio interface for IMT-2000 system (International Mobile Telecommunication system), which was standardized for use as the 3rd generation wireless mobile telecommunication system. It provides a variety of services such as voice services and multimedia mobile communication services in a flexible and efficient way. The standardization bodies in Japan, Europe, USA, and other countries have jointly organized a project called the 3rd Generation Partnership Project (3GPP) to produce common radio interface specifications for W-CDMA.
The standardized European version of IMT-2000 is commonly called UMTS (Universal Mobile Telecommunication System). The first release of the specification of UMTS has been published in 1999 (Release 99). In the mean time several improvements to the standard have been standardized by the 3GPP in Release 4, Release 5 and Release 6. A discussion on further improvements is ongoing under the scope of Release 7 and Study Item on Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN).
In the following, the legacy UTRAN Release 5 architecture and Radio Interface Protocol Architecture is described in detail.
UMTS Architecture
The high level Release 99/4/5 architecture of Universal Mobile Telecommunication System (UMTS) is shown in FIG. 1 (see 3GPP TR 25.401: “UTRAN Overall Description”, available from http://www.3gpp.org). The UMTS system consists of a number of network elements each having a defined function. Though the network elements are defined by their respective function, a similar physical implementation of the network elements is common but not mandatory.
The network elements are functionally grouped into the Core Network (CN) 101, the UMTS Terrestrial Radio Access Network (UTRAN) 102 and the User Equipment (UE) 103. The UTRAN 102 is responsible for handling all radio-related functionality, while the CN 101 is responsible for routing calls and data connections to external networks. The interconnections of these network elements are defined by open interfaces (Iu, Uu). It should be noted that UMTS system is modular and it is therefore possible to have several network elements of the same type.
FIG. 2 illustrates the current architecture of UTRAN. A number of Radio Network Controllers (RNCs) 201, 202 are connected to the CN 101. Functionally, the RNC 201, 202 owns and controls the radio resources in its domain and typically terminates the Radio Resource Control protocol on the access network side. Each RNC 201, 202 controls one or several base stations (Node Bs) 203, 204, 205, 206, which in turn communicate with the user equipments. An RNC controlling several base stations is called Controlling RNC (C-RNC) for these base stations. A set of controlled base stations accompanied by their C-RNC is referred to as Radio Network Subsystem (RNS) 207, 208. For each connection between User Equipment and the UTRAN, one RNS is the Serving RNS (S-RNS). It maintains the so-called Iu connection with the Core Network (CN) 101. When required, the Drift RNS 302 (D-RNS) 302 supports the Serving RNS (S-RNS) 301 by providing radio resources as shown in FIG. 3. Respective RNCs are called Serving RNC (S-RNC) and Drift RNC (D-RNC). It is also possible and often the case that C-RNC and D-RNC are identical and therefore abbreviations S-RNC or RNC are used. Commonly, a Drift RNS 302 is used for soft handovers of UEs between different RNS.
In the following, it will often be assumed that the C-RNC and D-RNC are coincident and in this case only abbreviations S-RNC or RNC will be used.
General Description of the Protocol Model of the UTRAN Terrestrial Interfaces
FIG. 4 shows an overview of the protocol model of the UTRAN in an UMTS network. For a better understanding, only a brief description is provided herein; further details may be found in Holma et al., “WCDMA for UMTS”, Third Edition, Wiley & Sons, Inc., October 2004, Chapter 5.
On the horizontal plane, the protocol model can be split into the radio network layer and the transport network layer. All UTRAN-related issues are visible and handled on the radio network layer, while transport network layer typically represents standard transport technology that is selected to be used for data transport for the UTRAN without any UTRAN-specific changes.
On the vertical plane, the protocol model can be split into control plane and user plane. The control plane is used for UMTS-specific control signaling (i.e. radio network layer related control signaling) and includes the Application Protocol (AP), e.g. RANAP on the Iu interfaces, RNSAP on the Iur interfaces, NBAP on the Iub and RRC on Uu interfaces. The control plane functions and Application Protocol allows setting up traffic radio bearers to the UEs via so-called signaling radio bearers.
While the control plane protocols are responsible for the UMTS-specific control signaling, the user plane transports the data streams sent by and sent to the users, such as voice calls, streaming data, packets of packet-switched services, etc. For transport, the user plane contains the so-called traffic radio bearers (also sometimes referred to as Data Bearers).
The transport network control plane is used for control signaling within the transport network layer and does not include any radio network layer related information. The transport network control plane includes the ALCAP protocol, which is used to set up the traffic radio bearers for exchanging user plane information and the signaling radio bearers required for communicating ALCAP protocol messages. Due to the presence of the transport network control plane, the Application Protocol within the control plane may operate completely independently from the technology selected for data transport on the traffic radio bearers in the user plane. The transport network control plane controls the operation of the transport network user plane.
UTRA Radio Interface Protocol Architecture
An overview of the radio interface protocol architecture of the UTRAN is shown in FIG. 5. Generally, the radio interface protocol architecture of the UTRAN implements Layers 1 to 3 of the OSI protocol stack. The protocols terminated in the UTRAN are also referred to as the access stratum (protocols). In contrast to the access stratum, all protocols not terminated in the UTRAN are typically referred to as the non-access stratum protocols.
As has been discussed with respect to FIG. 4, the vertical split of the protocols into user plane and control plane is illustrated. The Radio Resource Control (RRC) protocol is a Layer 3 protocol of the control plane which controls the protocols in the lower layers of the UTRA Radio Interface (Uu).
The RRC protocol is typically terminated in the RNC of the UTRAN, however other network elements have also been considered for terminating the RRC protocol in the UTRAN, e.g. the Node Bs. The RRC protocol is used for signaling of control information to control access to radio resources of the radio interface to the UEs. Further, there is also the possibility that the RRC protocol encapsulates and transports non-access stratum protocol messages, which are usually related to control within the non-access stratum protocol.
In the control plane, the RRC protocol relays the control information to Layer 2, i.e. the Radio Link Control (RLC) protocol, via signaling radio bearers through Service Access Points (SAPs). In the user plane the non-access stratum protocol entities may use traffic radio bearers to directly access Layer 2 via SAPs. The access may be made to the RLC directly or to the Packed Data Convergence Protocol which in turn provides its PDUs to the RLC protocol entity.
The RLC offers the SAPs to the higher layers. The SAPs define how RLC will handle the packets, e.g. whether RLC is operating in transparent, acknowledged or unacknowledged mode. The services provided to the higher layers in the control plane and user plane by the RRC or PDCP are also referred to as signaling radio bearer and traffic radio bearer, respectively.
The MAC/RLC layer in turn offers its services to the RLC layer by means of so-called logical channels. The logical channels essentially define what kind of data is transported. The physical layer offers its services to the MAC/RLC layer, the so-called transport channels. The transport channels define how and with which characteristics the data received from the MAC layer are transmitted via the physical channels.
Functional Distribution and Protocol Architecture
Functions of relevance to the invention that are located in the User Plane and Control Plane of the described network elements are exposed in the following.
RNC: Control Plane Functions
The Radio Resource Control (RRC) protocol on the network side is terminated in the RNC. It comprises, among others, the following functions that are relevant for the present invention:                Control of radio bearers, transport channels and physical channels,        Measurement control and processing of measurement reports,        RRC connection mobility functions.        
The Node B Application Part (NBAP) protocol, Radio Network Subsystem Application Protocol (RNSAP) and Radio Access Network Application Part (RANAP) protocol are terminated in RNC on Iub, Iur and Iu interfaces, respectively.
The following procedures of NBAP are used to support the previously mentioned RRC functions:                Radio Link Setup common NBAP procedure (NBAP)        Radio Link Addition/Deletion dedicated NBAP procedure (NBAP)        Radio Link Reconfiguration procedure (NBAP)RNC: User Plane Functions        
The RNC terminates the MAC and RLC protocols on the network side according to the Release 99/4 protocol architecture. User Plane protocol stack architecture of relevance to Release 5 High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA) will be exposed in the following. The settings of these protocols are controlled by the RRC.
Node B: User Plane/Control Plane Functions
Only the physical layer is terminated on the network side in Release 99/4 Node B. Release 5 Node B terminates the MAC layer for the High Speed-Dedicated Shared Channel (HS-DSCH) transport channel (MAC-hs), as will be explained later. Apart from terminating the NBAP protocol on the Iub interface, there is no control function in the Node B.
User Plane Protocol Stack Architecture in Case of HSDPA
The User Plane Radio Interface Protocol Architecture of HSDPA is shown in FIG. 6. The HARQ protocol and scheduling function belong to the MAC-hs sub-layer that is distributed across the Node B and UE. It should be noted that a selective repeat (SR) Automatic Repeat Request (ARQ) protocol based on sliding window mechanisms could also be established between the RNC and UE on the level of the RLC sub-layer in acknowledged mode. The service that is offered from the RLC sub-layer for point-to-point connection between the RNC and UE is referred to as a radio bearer.
Parameters of the protocols are configured by signaling in the Control Plane. This signaling is controlled by the Radio Resource Control (RRC) protocol for the signaling between radio network (S-RNC and UE) and by application protocols, NBAP on the Iub interface and RNSAP on the Iur interface.
User Plane Protocol Stack Architecture in Case of HSUPA
The User Plane Radio Interface Protocol Architecture of HSUPA is shown in FIG. 7. The HARQ protocol and scheduling function belong to the MAC-e sub-layer that is distributed across the Node B and UE. Reordering functions are necessary in soft handover in order to reorder data packets coming from different cells. These functions are located at the MAC-es at the S-RNC. As in HSDPA, parameters of the protocols are configured by signaling in the Control Plane. This signaling is controlled by the Radio Resource Control (RRC) protocol for the signaling between the radio network (S-RNC and UE) and by application protocols, NBAP on the Iub interface and RNSAP on the Iur interface.
Radio Bearer Establishment
Prior any transmission of data, the corresponding radio bearers are established and all layers are configured accordingly. The procedures for establishing radio bearers may vary according to the relation between the radio bearer and a dedicated transport channel. Depending on the Quality of Service parameters, there may or may not be a permanently allocated dedicated channel associated with a radio bearer.
In the following, an exemplary radio bearer establishment procedure with a dedicated physical channel activation in a legacy UMTS system will be explained based on FIG. 8.
The procedure shown in FIG. 8 is applied when a new physical channel needs to be created for a radio bearer. A radio bearer establishment is initiated when a RB Establish Request primitive is received from the higher layer Service Access Point on the network side of the RRC layer. This primitive contains a bearer reference and Quality of Service parameters. Based on these Quality of Service parameters, L1 and L2 parameters are chosen by the RRC entity on the network side. RRC determines the radio bearer parameters that are most appropriate for carrying data of the application/service based on the Quality of Service parameters of the application/service.
The physical layer processing on the network side is started with the CPHY-RL-Setup request primitive issued to all applicable Node Bs. If any of the intended recipients is unable to provide the service, it will be indicated in a confirmation primitive. After setting up L1, including the start of Transmission/Reception in Node B, the NW-RRC sends a RADIO BEARER SETUP message to its peer entity (acknowledged or unacknowledged transmission optional). This message contains L1, MAC and RLC parameters. For example, at radio bearer setup, each involved logical channel is configured with a MAC logical channel priority in the range of 1 to 8. The MAC logical channel Priority is contained in the information element RB mapping info. After receiving the message, the UE-RRC configures L1, MAC and RLC in accordance to the signaled parameters.
When L1 synchronisation is indicated, the UE sends a RADIO BEARER SETUP COMPLETE message in acknowledged-mode back to the network. The NW-RRC configures MAC and RLC on the network side. After receiving the confirmation for the RADIO BEARER SETUP COMPLETE, the UE-RRC creates a new RLC entity associated with the new radio bearer. The applicable method of RLC establishment may depend on RLC transfer mode. The RLC connection can be either implicitly established, or explicit signaling can be applied. Finally, an RB Establish Indication primitive is sent by the UE-RRC and an RB Establish Confirmation.
It is also possible to change radio bearer properties in the course of an active connection. The radio bearer reconfiguration procedure is used to reconfigure parameters for an already established radio bearer.
Evolved UTRA Radio Interface Protocol Architecture
Currently, evolutions of the radio interface as well as the radio network architecture are investigated in the recently introduced study item “Evolved UTRA and UTRAN” in 3GPP. The aim of the study item is to ensure competitiveness of the 3GPP radio-access technology in a long time frame. The targeted goals include reduced latency, higher user data rates, optimised support for packet services, improved system capacity and coverage, and reduced cost for the operator, while reducing system complexity.
The current architecture is shown in FIG. 9, where the RRC is fully located at the RNC and the layer 2 (MAC sub-layers and RLC) are distributed over the Node B and the RNC. Several different proposals on the architecture are currently discussed. One of the proposals is illustrated in FIG. 10. The proposal consists in moving the complete layer 2 (MAC and RLC) and maybe part of the RRC functionality, that reside in the legacy UMTS Release 6 system in the RNC, towards the air interface, i.e. to the Node B, in order to reduce latency. Another proposal under discussion is shown in FIG. 11, wherein the legacy RNC in the evolved UTRAN architecture is removed and the functionalities formerly located in the RNC are distributed across the Node B and SGSN (Serving GPRS Support Node), which is a part of the Core Network.
Resource Allocation Techniques in Orthogonal Frequency
Division Multiplexing Access
OFDMA, also referred to as Multi-user OFDM, is being considered as a modulation and multiple access method for future generation wireless networks. OFDMA is an extension of Orthogonal Frequency Division Multiplexing (OFDM), which is already employed in data access systems such as IEEE 802.11 wireless LAN (WiFi) and IEEE 802.16 wireless broadband access systems (WiMAX).
The basic principle of Orthogonal Frequency Division Multiplexing (OFDM) is to split the frequency band into a number of narrowband channels. Therefore, OFDM allows transmitting data on relatively flat parallel channels (sub-carriers) even if the channel of the whole frequency band is frequency selective due to a multi-path environment. Since the sub-carriers experience different channel states, the capacities of the sub-carriers vary and permit a transmission on each subcarrier with a distinct data-rate. Hence, sub carrier-wise (frequency domain) Link Adaptation by means of Adaptive Modulation and Coding increases the radio efficiency by transmitting different data-rates over the sub-carriers. OFDMA allows multiple users to transmit simultaneously on the different sub-carriers per OFDM symbol. Since the probability that all users experience a deep fade in a particular sub-carrier is very low, it can be assured that sub-carriers are assigned to the users who see good channel gains on the corresponding sub-carriers. When allocating resources in the downlink to different users in a cell, the scheduler takes information on the channel status experienced by the users for the sub-carriers into account. The control information signaled by the users, i.e. CQI, allows the scheduler to exploit the multi-user diversity, thereby increasing the spectral efficiency.
Two different resource allocation methods can be distinguished based on the radio access scheme that distributes available frequency spectrum among different users in OFDMA. The first allocation mode, called localized mode, tries to benefit fully from frequency scheduling gain by allocating the sub-carriers on which a specific UE experiences the best radio channel conditions. Since this scheduling mode requires associated signaling (resource allocation signaling, CQI in uplink), this mode would be best suited for non-real time, high data rate oriented services. In the localized resource allocation mode a user is allocated continuous blocks of sub-carriers.
The second resource allocation mode, called distributed mode, relies on the frequency diversity effect to achieve transmission robustness by allocating resources that are scattered over time and frequency grid. The fundamental difference with localized mode is that the resource allocation algorithm does not try to allocate the physical resources based on some knowledge on the reception quality at the receiver but select more or less randomly the resource it allocates to a particular UE. This distributed resource allocation method seems to be best suited for real-time services as less associated signaling (no fast CQI, no fast allocation signaling) relative to the localized mode is required.
The two different resource allocation methods are shown in FIG. 12 for an OFDMA based radio access scheme. As can be seen from the left-hand part of the figure, which depicts the localized resource allocation mode, the localized mode is characterized by the transmitted signal having a continuous spectrum that occupies a part of the total available spectrum. Different symbol rates (corresponding to different data rates) of the transmitted signal imply different bandwidths (time/frequency bins) of a localized signal.
On the other hand, as can be seen from the right-hand part of the figure, the distributed resource allocation mode is characterized by the transmitted signal having a non-continuous spectrum that is distributed over more or less the entire system bandwidth (time/frequency bins).
Link Adaptation Methods
Wireless systems in opposition with fixed network systems are by nature fast time-varying systems. This means that the physical channel state varies significantly over time, which makes data transfer very unreliable. For instance, in a coaxial cable or fiber optic, the level of interference is well known and only slowly changing. In wireless systems, the characteristics of the air interface change on a much quicker basis (e.g. few ns) depending on the mobile station speed, and the propagation models. In order to combat this, encoding techniques are used in order to recover from losses due to fading deeps. Although powerful codes exist, high data rates demand for further protection mechanism.
One such power control technique deployed in GSM, but implemented in particular in UMTS, relies on adapting the transmission power to the receive power. With this procedure, the transmission unit tries to compensate for fading deeps experienced by the receiver. The main drawbacks of power control techniques are that they require very fast and reliable feedback signals and tend to increase complexity of the transmission and reception units. Furthermore, bad implementation of power control algorithms may have significant impact on the overall system performance as unnecessary interference is generated.
Several link adaptation techniques exist for HSDPA (High Speed Downlink Packet Access), which is a feature that has been added in the Release 5 specifications of the UMTS standard. The aim of HSDPA is to provide means to support high data rate services in the downlink. Several new techniques have been introduced in order to meet the requirements of HSDPA.
A further adaptation technique is Adaptive Modulation and Coding (AMC), which has been introduced with HSDPA in Release 5. Instead of trying to compensate for fading deeps by varying the transmission power, the code rate and the used modulation (e.g. QPSK or 16QAM) are selected depending on channel quality feedback messages sent from the receiving unit to the transmission unit. These channel quality indicator messages (CQI) indicate the code rate the receiver unit could receive with a given probability under the same radio conditions. Should the radio conditions be poor, the receiver would recommend a small code rate, I.e. more encoding. On the other hand, when radio conditions are good, the receiver recommends a small code rate, I.e. little encoding, which enables the transmission of more data during the same period. The different levels available form a set called Modulation and Coding Set. Furthermore, it shall be well understood that the CQI messages only give information on the measured radio conditions and not on the experienced decoding quality at the receiver side.
Another adaptation technique is Node B controlled scheduling, wherein CQI messages are used by HSDPA system for the scheduling of users in a cell. Depending on the received CQIs from a population of UEs and on the scheduling algorithm used in the Node B (MAC-hs), the available physical resources are distributed among the different users. For instance, the complete set of resources can be allocated to the UE with the best CQI. This method provides the best overall system throughput in the cell, however at the cost of unfairness, as it tends to schedule only UEs that are close to the Node B and thus UEs at the cell edge would be given no service.
A further common technique for error detection and correction in packet transmission systems over unreliable channels used in HSDPA is called Hybrid Automatic Repeat reQuest (HARQ). Hybrid ARQ is a combination of Forward Error Correction (FEC) and ARQ.
If a FEC encoded packet is transmitted and the receiver fails to decode the packet correctly (errors are usually checked by Cyclic Redundancy Check bit), the receiver requests a retransmission of the packet. Depending on the information (generally code-bits/symbols), of which the transmission is composed of, and depending on how the receiver processes the information, the following HARQ schemes are defined:
Type I:
If the receiver fails to decode a packet correctly, the information of the encoded packet is discarded and a retransmission is requested. This implies that all transmissions are decoded separately. Generally, retransmissions contain identical information (code-bits/symbols) to the initial transmission.
Type II:
If the receiver fails to decode a packet correctly, a retransmission is requested, where the receiver stores the information of the erroneously received encoded packet as soft information (soft-bits/symbols). This implies that a soft-buffer is required at the receiver. Retransmissions can be composed out of identical, partly identical or non-identical information (code-bits/symbols) according to the same packet as earlier transmissions. When receiving a retransmission the receiver combines the stored information from the soft-buffer and the currently received information and tries to decode the packet based on the combined information. Type II schemes are more sophisticated than Type I schemes, since the probability for correct reception of a packet increases with receive retransmissions. This increase comes at the cost of a required hybrid ARQ soft-buffer at the receiver.
HARQ type II can be performed in two different ways: Chase combining or incremental redundancy. In Chase combining, each retransmission repeats the first transmission or part of it. In incremental redundancy, each retransmission provides new code bits from the mother code to build a lower rate code. While Chase combining is sufficient to make Adaptive Modulation and Coding robust, incremental redundancy offers the potential for better performance with high initial and successive code rates, at higher Signal to Noise Ratio estimation error and FER operating points (i.e a greater probability that a transmission beyond the first will be needed), albeit at the cost of additional memory and decoding complexity. Another drawback of incremental redundancy is that some transmission version may not be decodable by the receiver if considered alone (non-self-decodable retransmissions). Indeed, a systematic turbo encoded data packet contains the original information bits (systematic bits) and additional parity bits (redundancy).
Type III:
This is a subset of Type II with the restriction that each transmission must be self-decodable.
The present invention is related in particular to Type II and Type III schemes, wherein the received transmissions are soft-combined.
Traditional HARQ schemes as presented above only require a single bit of feedback to the transmitter per transmission indicating a successful decoding, usually called ACK (Acknowledged), or a decoding failure, usually called NACK (Not Acknowledged). Intensive research has been done in order to further enhance the performance of an HARQ system. One research area that has been investigated is HARQ type II schemes based on received packet reliability, wherein reliability information of the received packet computed by the receiver is taken into account for shaping the retransmission of unsuccessful packets. One possible method of generating this reliability information is to compute the average magnitude of the log-likelihood ratios of the information bits at the output of the decoder. This information can be used to indicate to the transmission unit, along the NACK signal, the amount and eventually the position of the bits that should be retransmitted.
The main problem with this approach is that the amount of signaling required by such a method would be prohibitive. So as to simplify this method, instead of indicating a specific number of bits to retransmit, multiple NACK levels may be used in order to signal a level of decoding to the transmitter (e.g. 1, 2, 3). The multi level NACK messages are required to be interpreted by the transmitter side, therefore the number of levels and their exact definition of the multi level NACK messages must be known at both the receiver side and at the transmitter side. Multi level NACK messages can be either L1 messages by mapping the different levels onto the different constellation points on multilevel modulation techniques (e.g. QPSK, 16QAM) or L2 messages. In the second options, the different levels are represented by different codewords (e.g. 1st level: 00001, 2nd level: 00010, 3rd level: 00100, etc.). Moreover, the reliability information can be optionally jointly encoded with other information, e.g. CQI, and the resulting encoded data can be CRC (Cyclic Redundancy Check) protected. The benefit of a L2 messages is the increase reliability of the messages. However, encoding/decoding process and CRC check are computation expensive processes with some processing delay.
A further simplification has been proposed in the form that the receiver has only in principle two levels to indicate the non-decoding of a packet (NACK and LOST), where LOST indicates a severe corruption of the received packet to such an extent that no combination with future retransmission is envisaged by the receiver. In this case, the transmitter should retransmit a self-decodable version of the packet. The benefit of this technique is its simplicity and its robustness, but the transmitter is not given any information on how to shape retransmission in case a NACK is transmitted from the receiver.
As previously mentioned, a multi level NACK feedback is beneficial since it allows the HARQ protocol transmitting entity to adjust packet re-transmissions based on the decoding quality of previously received transmissions of a data packet. This in turn improves the transmission efficiency and the overall data throughput.
However, the generation of accurate multi level NACK signals requires extensive computation at the receiving side. This tends to increase the power consumption at the receiving entity, which may be a critical issue if the receiving entity is a mobile terminal. The increase in computation requirements also tends to increase the processing delay at the receiving side, which may become a burden for services having a very tight delay requirement, such as real-time services, and may set the upper limit for the achievable data rate. Indeed, a very fast acknowledgment mechanism enables the transmitter to process more packets within a given time.
Further, a multi-level NACK feedback re-transmission protocol is by nature less reliable than a simple ACK/NACK feedback protocol, when assuming the same transmission power. Indeed, instead of having to distinguish between two values (ACK or NACK), the transmitter side has to separate several values (e.g. ACK, NACK—1, NACK—2, NACK—3). For the same transmission power of the feedback messages, the probability of a misinterpretation of the feedback levels is higher compared to a conventional ACK/NACK feedback scheme. Reliability issues are critical since a misinterpretation of a feedback message directly leads to a performance loss. Especially NACK to ACK and ACK to NACK misinterpretations are severe as they create protocol errors that need to be detected and corrected by special functions (e.g. RLC in HSDPA). Under normal circumstances, the transmission unit should be able to decode the NACK signals with a sufficiently good reliability. However, for example in case of a sudden increase of the cell interference, the multi level NACK decoding error probability increases as well.
In a similar way, UEs that are at the cell edge or UEs which are power limited due to other simultaneous uplink data traffic may not have enough power available in order to transmit the multi level NACK messages with the required power. By transmitting the feedback messages at a reduced power level, the probability of an erroneous decoding of the HARQ feedback at the Node B is increased. It should be noted that this effect applies for both means of transmitting HARQ feedback messages, i.e. L1 signals or L2 signals, although L1 signals tend to be more sensitive to this issue.
Consequently, although multi level NACK signaling brings benefits in terms of achieved system throughput, the associated drawbacks may for some services, such as real-time services or services with small packets or under some circumstances, e.g. limited transmission power, counter-balance the potential benefits.